1. Field
Electrical power conversion and more specifically an AC power exchanger that can adapt to the variable voltage and frequency characteristics from an electrical power grid source and generate a stable power output that could be used to power common household appliances designed to operate at the output power.
2. General Background and State of the Art
Many appliances such as washing machines, dryers, dishwashers and the like required generally stable power source, which in the United States would 120 VAC, 60 Hz power. This arises at least in part because the timing mechanism of the appliance is dependent on the frequency of the electrical power used to control the appliance. For example, the appliance will detect the frequency of the electrical power and use that frequency as a “clock” signal against which the sequencing and duration of various automatic operations will be set. Consequently, appliances which are made to operate on 60 Hz power will not function properly if the power is a different frequency such as 50 Hz. Other appliance functions such as driving the motor also require the 60 Hz output. The same is true for appliances manufactured to operate at 50 Hz when the source power is 60 Hz.
The conversion of power from 220 VAC, 50 Hz was described in U.S. Pat. No. 5,267,134. However, that patent did not take into account the reality of the variable frequency and voltage of the typical international power source and specifically the need to accept power from a power source that varied unpredictably, for example, around a 220 VAC nominal characteristic. In such cases, the voltage can vary from 180 VAC to as high as 300 VAC. As such, adaptors in consumer applications must be able to adapt to the input power and generate an output power that is voltage regulated with minimum of sine wave distortions and harmonics and be surge protected.
It has also been found that converters suitable for the consumer applications must maintain conventional power capabilities (about 15 amps) yet at the same time meet several often-conflicting requirements that preclude conventional approaches. For example, the adaptor must be compact to be able to fit physically in the limited spaces available to a consumer. Conventional converters are inherently large and bulky and are ill-suited for consumer applications. Also, consumer environments are subject to contamination by moisture (humidity), dust, lint, articles of clothing and the like that can seriously degrade the performance of the adaptor electronics, particularly in compact environments. Consequently, the adaptor electronics must be sealed for protection from such contaminants and obstructive items. Such sealing precludes the use of internal fans for cooling, a conventional approach to cooling converters. Therefore, passive conduction cooling of sufficient capacity must be devised. The adaptor must also be lightweight, preferable not more than ten to twelve pounds. Conventional converters incorporate heavy inductors and transformers and other components that result in converters that weigh as much as 50-60 pounds or more. In many environments, the conventional converter had to function reliably in high ambient temperature conditions thereby requiring noisy, robust active cooling systems.
Such approaches are incompatible with the requirements of consumer adaptors particularly where such adaptors are in enclosed or sealed environments where cooling is limited to reliance on passive cooling. Therefore, novel circuits and component selections were conceived to increase circuit efficiency from about 87% to 93% or more to reduce heat generation to a point where passive heat dissipation methods were sufficient to dissipate heat while maintaining output power.
A need, therefore, exists for an compact, lightweight and powerful adaptive AC power exchanger useful in a consumer environment that will accept unpredictably variable AC power available from a power utility grid and adapt that power to provide AC power at a stable, predictable level, for example, the 120 VAC, 60 Hz used by common household appliances made for the United States. In addition, there is a need to provide a converter that provides 15 amp power, which is the common current standard for most home circuits throughout the world. The variability of the power in many countries can include extended periods of time when there is no power available. The restoration of power may result in power surges that can cause damage to any connected appliance. Therefore, any converter should be able to accept large inrush currents for a brief period when power is restored after an outage, without damage to the converter circuitry while also protecting the connected appliance from such power surges. In addition, because the voltage can vary for example from as low as 180 VAC to as high as 280 VAC or even as high as 300 VAC for brief periods, the circuitry of any converter should be able to function as a voltage regulator to insure that the voltage output is maintained substantially at about 120 VAC.
There also is a need for a suitable converter to be able to function reliably in adverse environments, which may include dust or humidity, by sealing the housing containing the converter electronics. This also means that all internal moving parts including internal cooling fans be eliminated and heat dissipated without using active cooling such as fans in the enclosed converter circuitry housing.
Using conventional electronics in the consumer environment and applications described, additional problems arise that contribute to inefficiencies. For example, no load power losses contribute to significant performance inefficiencies. Specifically, when the AC input power is 220V, the VDC will be about 400 VDC. However, the suitable VDC for 120 VAC output is only 200 VDC. To provide 400 VDC will cause high circulation current on the circuit inductors and capacitors, which will produce switching loss and conduction loss. The high no load loss is a serious energy loss for home application as the load level would likely be very low most of time. Reducing that energy loss will also reduce heat generated and hence the need for passive instead of active heat dissipation.
A second problem is high power losses when the load level is high. Specifically, high VDC at the output of the A-D converter will require higher voltage ratings for IGBTs and MOSFETs and a higher inductance values for certain inductor components. Higher voltage IGBTs and MOSFETs result in conduction and switching losses in high current (high load) applications. The high inductance inductors also have high resistance resulting in high power losses in high load applications.
A third problem relates to high surge current loads, which can occur when high load appliances such as refrigerators, air-conditioners, vacuum cleaners and the like are attached. In such applications, the peak current on the inductors in response to the high surge current will cause saturation of the inductors, which in turn causes loss of inductance rendering the circuit vulnerable to damage.
A fourth issue relates damage that can occur in abnormal circumstances where a high DC voltage passes through the converter to the output, which can cause damage to the connected appliance (load) itself.